This invention relates to a system of pre-made reinforcement devices, commonly known as post-and-core systems, which fit into root-canal-treated teeth. This invention is applicable to all root canal treated teeth where the filling or obturation of the root canal by a dentist is done with modern methods and materials which use a thermoplastic or injection technique.
The purpose of the post in any post-and-core system in a root-canal-treated tooth is to reinforce the root structure about all three axes of rotation (horizontal, vertical and sagittal) when the root-canal-treated tooth is subjected to stress from the forces of mastication (chewing forces). The purpose of the core in any post-and-core system restoring a root-canal-treated tooth is to provide a tapered retention vehicle above a prepared root such that a dental fixed prosthesis will be retained when a dental cement is used. When the post-and-core is cemented into the root of the root-canal-treated tooth, they act together with the dental crown as one complete unit. The post, core and crown complex will act like, look like, and withstand forces as any virgin or non-root-canal-treated tooth. Ever since the development of techniques for filling root-canal-treated teeth, dentists have been searching for a simple, easily utilizable, and most important, a dependable design of post-and-core system to provide the "male" portion wherein the root would be the "female" portion.
All post-and-core systems are held in the prepared root by a dental cement in conjunction with a long post. The ideal length of the post is calcualted after measuring of the dental radiograph the distnce in millimeters from the apex of the tooth to the level of the interproximal bone. When using a "parallel-post" technique, the post should extend at least 6 millimeters apically from the coronal extent of the parallel preparation.
The post-hole in the root of the root-canal-treated tooth is prepared and shaped by any of several commercially available dental drills designed specifically for this purpose. The post-hole is slightly larger in diameter than the post. This is necessary to allow room for the cement and to prevent the post from engaging the sides of the walls of the post-hole preparation. Because a root-canal-treated tooth is prone to fracture, any excessive stress, such as the frictional stress created if the post were slightly larger than the post-hole, could create possible stress lines in the lateral walls of the post-hole preparation. The post is usually vented to allow the escape of dental cement as the post is inserted into a cement-filled post-hole. This venting reduces the likelihood of potential fracture lines being created in the apical end of the root due to compression of the dental cement.
Until recently, all techniques available for preparation of a post-and-core either had limitations with regard to accepting a practical post-and-core fabrication at the expense of reduced physical properties or producing an ideal post-and-core with maximum physical properties but with a technique that is demanding on the time and effort required by the dentist and on the financial investment required by the patient. The practical fabrication described above would include the use of dental amalgam or composite resin to form a core with a pre-made stock cylindrical post.
The other alternative available to the dentist would be a cast post-and-core wherein the post-and-core is fabricated first in wax or plastic either on a dental model or directly in the patient's mouth. Using this approach, the plastic or wax pattern is then processed into a metal post-and-core by a lost-wax casting process.
The core buildups of dental amalgam and composite resin have certain inherent limitations. The dental amalgam and composite resin core buildups condensed against commercially available pre-made cylindrical posts have certain weaknesses with regard to their limited strength when subjected to the forces of mastication. These forces placed on a tooth are in three planes and exert pressure on teeth in an anterior-posterior direction, a side-to-side direction, and an up-and-down direction, or any combination of these directions.
These facts are important because a technique will be described later which will prevent any post in a post-and-core system from rotating in a clockwise or counterclockwise direction. A significant percentage of failures in post-and-core systems takes place at the junction of the post-and-core and the tooth. Many of these failures are due to an inability of the post-and-core to resist rotational forces, also known as shearing forces.
It is appropriate at this point to describe the limitations of modern dental cements which are used by dentists. Most dental cements work on an ability to enhance physical interlocking of two surfaces in close approximation. There is significant evidence in professional literature that no true chemical bond exists between dental cement (regardless of the chemical formulation) and stainless steel posts, dental crowns, root canal post-hole preparations, or the lateral walls of the crown preparation of a non-root-canal vital tooth.
Many attempts at correcting failures of pre-made post-and-core systems have been made, but none have completely resolved two basic reasons for many post-and-core system failures. One reason for many failures is that the post bends due to stress from the forces of mastication. This problem can be solved by using a thicker post, i.e., a post with a larger diameter to give more thickness and strength to the post. Another significant reason for failures is the dislodgement of the post-and-core under the influence of rotational forces which place the cement union of the post-core and the tooth under shearing stress. Attempts at solving this latter problem include the use of retention pins in the core and the use of screw-in posts where the post is larger in diameter than the post-hole.
The use of commercially available retention pins for retaining dental amalgam or composite resin cores provides additional retention for the dental amalgam or composite resin core but does little to resist shearing forces. An additional problem with the retention pins is that their use may set up potential stress or fracture lines in the root-canal-treated tooth. In addition to doing very little to prevent a failure of the interface of the post and the core, the placement of retention pins presents a clear and persistent danger of precipitating a clinical fracture of the tooth.
The use of screw-in posts has been proposed as a solution to counteract the disloding effect of shearing forces. With this system, the post is larger than the post-hole. The post is wedged or screwed in the post-hole preparation, with or without dental cement. The placement of the screw-in post places the lateral walls of the post-hole preparation under tension. The use of the screw-in post is questionable because the root of the root-canal-treated tooth is readily prone to fracture. A split tooth is a common problem associated with the use of a screw-in post. Also, the screw-in post only counteracts the effect of rotational forces in one direction. Assuming the screw-in post is screwed in and not placed in the post-hole preparation with a mallet, the dentist uses a clockwise rotational motion to advance the post into the post-hole. There is nothing in the design of a screw-in post, once it is seated completely, to oppose a counterclockwise force which tends to dislodge this type of post.
The present invention addresses many of the problems inherent in other pre-made post-and-core systems and a workable and practical solution will be described.
Accordingly, it is an object of the present invention to provide a pre-made system for reinforcing a root-canal-treated tooth by means of an anti-rotational device.
A further object of the present invention is to provide a pre-made post-and-core system which will eliminate clinical problems associated with currently available premade post-and-core systems.
A still further object of the present invention is to include a horizontal plane anti-rotational device which places dental cements under compressive rather than shearing forces when subjected to horizontal plane clockwise-counterclockwise forces from mastication.